Enqueue Operation in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enqueue operation on a queue implemented with a list

What is the output of the queue after performing the enqueue operation in the following code?

DSA Python

queue = [1, 2, 3]
queue.append(4)
print(queue)

A [4]

B [4, 1, 2, 3]

C [1, 2, 3, 4]

D [1, 2, 3]

💡 Hint

Remember that enqueue adds an element to the end of the queue.

The append method adds the new element 4 to the end of the list representing the queue.

❓ Predict Output

intermediate

Output of enqueue operation on a queue implemented with collections.deque

What is the output of the queue after performing the enqueue operation in the following code?

DSA Python

from collections import deque
queue = deque([10, 20, 30])
queue.append(40)
print(list(queue))

A [10, 20, 30]

B [10, 20, 30, 40]

C [40, 10, 20, 30]

D [30, 20, 10, 40]

💡 Hint

The append method of deque adds to the right end.

The append method adds 40 to the right end of the deque, preserving order.

🔧 Debug

advanced

Identify the error in enqueue operation on a linked list queue

What error will the following code raise when trying to enqueue an element?

DSA Python

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class Queue:
    def __init__(self):
        self.front = None
        self.rear = None

    def enqueue(self, data):
        new_node = Node(data)
        if self.rear is None:
            self.front = new_node
        else:
            self.rear.next = new_node
        # Missing update of rear pointer

q = Queue()
q.enqueue(5)
q.enqueue(10)
print(q.front.data, q.rear.data)

A AttributeError: 'NoneType' object has no attribute 'data'

B None None

C TypeError: unsupported operand type(s) for +: 'int' and 'Node'

D 5 10

💡 Hint

Check if rear pointer is updated after enqueue.

The rear pointer is never updated after adding a new node, so rear remains None, causing AttributeError when accessing rear.data.

❓ Predict Output

advanced

Output of enqueue operation with circular queue array implementation

Given a circular queue of size 5 with front=3, rear=4 and elements [_, _, _, 10, 20], what is the queue state after enqueueing 30?

DSA Python

size = 5
queue = [None, None, None, 10, 20]
front = 3
rear = 4
rear = (rear + 1) % size
queue[rear] = 30
print(queue)

A [30, None, None, 10, 20]

B [None, 30, None, 10, 20]

C [None, None, 30, 10, 20]

D [None, None, None, 10, 20]

💡 Hint

Remember to wrap around using modulo for circular queue.

rear moves from 4 to 0 (wrap around), so queue[0] = 30.

🧠 Conceptual

expert

Maximum number of elements after enqueue operations in a fixed-size circular queue

A circular queue has size 7. Initially, it is empty. After performing 5 enqueue operations and 3 dequeue operations, what is the maximum number of elements that can be in the queue?

A 2

B 7

C 5

D 4

💡 Hint

Think about how many elements remain after dequeues.

After 5 enqueues, queue has 5 elements. After 3 dequeues, 2 elements are removed, so 3 remain. But maximum number after these operations can be 4 if enqueue and dequeue interleave differently.